In conjunction with the control system for a motor vehicle such as for a power steering system, an engine ignition control system and others, a control system for the motor vehicle which is designed for correcting an error peculiar to the system by resorting to the use of learning function is well known in the art, as is disclosed, for instance, in Japanese Unexamined Patent Application Publication No. 47471/1991 (JP-A-3-47471). However, in the control system for the motor vehicle equipped with such learning function, it is difficult to ensure reliability for the correcting control with high accuracy because of difficulty encountered in setting the conditions for learning.
Under the circumstances, there have been proposed a variety of apparatuses for adjusting variances of characteristics of individual systems in the course of assembling on the production line for ensuring reliability of correction with high accuracy.
In the following, adjustment of dispersion or variance of the characteristic of the control system for the motor vehicle as carried out on the production line and known heretofore will be described by taking as an example a motor-driven power steering control system.
FIG. 7 is a circuit diagram showing a conventional motor-driven power steering control system with several parts being shown in blocks.
Referring to FIG. 7, a steering effort assisting motor 40 (output unit) is electrically driven, being supplied with a motor current IM from a battery 41, for thereby generating an assist torque to be applied to a steering wheel (not shown) of a motor vehicle.
Ripple components of the motor current IM is absorbed by a capacitor 42 of a large capacity (on the order of 1000 .mu.F. to 3600 .mu.F.), wherein the motor current IM is detected by means of a shunt resistor 43. One terminal of the capacitor 42 is connected to the ground potential by way of a wiring conductor L1.
Further, the motor current IM is changed in dependence on magnitude and direction of the assist torque by means of a bridge circuit 44 which is constituted by a plurality of semiconductor switching elements (e.g. FETs) Q1 to Q4.
The semiconductor switching elements Q1 to Q4 cooperate to constitute the bridge circuit 44, being interconnected in the form of a bridge circuit by wiring conductor patterns P1 and P2.
The bridge circuit 44 is connected to the shunt resistor 43 via the wiring conductor patterns P1 and P2. Further, the output terminal of the bridge circuit 44 is realized by a wiring conductor pattern P3.
The motor 40 and the battery 41 are connected to the bridge circuit 44 by means of a connector 45 having a plurality of lead terminals. The motor 40, the battery 41, and the connector 45 are interconnected by external wiring conductors L2. The motor current IM can be interrupted by means of a normally opened relay 46 as occasion requires.
The relay 46, the capacitor 42 and the shunt resistor 43 are interconnected by a wiring conductor pattern P4. The connector 45 is connected to the ground potential by a wiring conductor pattern P5. The wiring conductor pattern P3 constituting the output terminals of the bridge circuit 44 is connected to the connector 45.
The motor 40 is driven by a driving circuit 47 by way of the bridge circuit 44. The driving circuit 47 is connected to an excitation coil of the relay 46 by way of a wiring conductor L3 for driving the relay 46. Further, the driving circuit 47 is connected to the bridge circuit 44 via wiring conductors L4.
The motor current IM is detected by a motor current detecting means 48 as a voltage appearing across the shunt resistor 43. The driving circuit 47 and the motor current detecting means 48 constitute peripheral circuit elements of a microcomputer 55 which will be described hereinafter.
A steering torque T applied to the steering wheel is detected by a torque sensor 50, while a speed V of a motor vehicle is detected by a vehicle speed sensor 51.
The microcomputer 55 constitutes an electronic control unit (ECU) in cooperation with input/output control units (input/output interfaces) for arithmetically determining the assist torque on the basis of the steering torque T and the vehicle speed V while generating a rotating direction command Do and a current control quantity Io for controlling the bridge circuit 44 as a driving signal which corresponds to the assist torque and which is derived from a feedback signal indicating the motor current IM, wherein the rotating direction command Do and the current control quantity Io are outputted to the driving circuit 47.
The microcomputer 55 includes a motor current determining means 56, a subtracting means 57 and a PID (proportional-integral-differential) arithmetic means 58.
The motor current determining means 56 is designed to generate the rotating direction command Do for the steering effort assisting motor 40 and a motor current command Im equivalent to the assist torque, while the subtracting means 57 is designed to arithmetically determine a current deviation .DELTA.I of the motor current IM from the motor current command Im.
The PID arithmetic means 58 arithmetically determines correcting quantities for the P (proportional) term, the I (integral) term and the D (differential) term, respectively, on the basis of the current deviation .DELTA.I, to thereby generate a current control quantity Io corresponding to a PWM (Pulse-Width Modulation) duty ratio.
Further, in addition to an A/D converter, a PWM timer circuit and others, the microcomputer 55 includes a self-diagnosis function known per se for carrying out constantly the self-diagnosis as to whether or not the system is operating normally, wherein upon occurrence of abnormality, the relay 46 is opened by way of the driving circuit 47 to thereby interrupt the motor current IM. The microcomputer 55 is connected to the driving circuit 47 by wiring conductors L5.
Next, description will be directed to operation of the conventional motor-driven power steering control system shown in FIG. 7.
At first, the microcomputer 55 fetches the steering torque T and the vehicle speed V from the outputs of the torque sensor 50 and the vehicle speed sensor 51, respectively, while fetching the motor current IM from the shunt resistor 43 as a feedback input quantity, to thereby arithmetically determine the rotating direction command Do and the current control quantity Io corresponding to the magnitude of the assist torque for the power steering on the basis of the steering torque T, the vehicle speed V and the motor current IM, wherein the rotating direction command Do and the current control quantity Io as determined are outputted to the driving circuit 47 via the wiring conductors L5.
In the steady driving state, the normally opened relay 46 is closed by the driving circuit 47 in response to the command supplied through the wiring conductor L3. However, upon inputting of the rotating direction command Do and the current control quantity Io, the PWM driving signals are generated to be applied to the individual semiconductor switching elements Q1 to Q4, respectively, of the bridge circuit 44 via the wiring conductors L4.
Thus, the motor current IM is supplied to the motor 40 from the battery 41 by way of the external wiring conductors L2, the connector 45, the relay 46, the wiring conductor pattern P4, the shunt resistor 43, the wiring conductor pattern P1, the bridge circuit 44, the wiring conductor pattern P3, the connector 45 and the external wiring conductors L2. The motor 40 is then driven by the motor current IM to generate the assist torque of demanded magnitude in the direction as demanded.
In that case, the motor current IM is detected through the medium of the shunt resistor 43 and the motor current detecting means 48 to be fed back to the subtracting means 57 incorporated in the microcomputer 55, whereby the motor current IM is so controlled as to coincide with the motor current command Im. Incidentally, the motor current IM contains ripple components due to switching operations involved in the PWM driving of the bridge circuit 44. However, the ripple components are suppressed by the smoothing capacitor 42 of a large capacity, to smooth the motor current.
At this juncture, it should be mentioned that the value of the motor current IM controlled by the motor-driven power steering control system of this type is relatively large on the order of 25 amperes even in the case of a low-horse-power motor vehicle and within a range of 60 to 80 amperes in the case of a small-size motor vehicle. Besides, in order to suppress variation or fluctuation of the assist torque, high accuracy is required for the control of the current value.
However, due to variance of the characteristics of the shunt resistor 43 and the parts constituting the motor current detecting means 48, the demanded accuracy of the current value can not be realized without resorting to adjustment. Such being the circumstances, accuracy alignment has heretofore been performed by adjusting the motor current IM through a motor current adjusting process on a production line on a vehicle-by-vehicle basis.
Next, by referring to a circuit diagram shown in FIG. 8 together with FIG. 7, description will be made of adjusting operation or procedure of the motor current IM performed heretofore.
FIG. 8 shows in concrete a circuit arrangement of the motor current detecting means 48 shown in FIG. 7.
Referring to FIG. 8, the motor current detecting means 48 is composed of a comparator CM for comparison of the voltage appearing across the shunt resistor 43, a resistor R1 connected to an input terminal of the comparator CM, an adjusting resistor RA connected in parallel to the resistor R1, a transistor TR operating in response to the output level of the comparator CM and an output resistor Ro inserted between the collector of the transistor TR and the ground potential, wherein a detection signal corresponding to the motor current IM is generated on the basis of the voltage appearing across the shunt resistor 43 to be outputted.
Parenthetically, for the adjusting of the motor current IM, there are employed a measuring unit and a motor current adjusting unit (both not shown) which are provided separately from the microcomputer 55.
For carrying out the adjustment of the motor current, a predetermined pseudo-signal is inputted to the microcomputer 55 from the torque sensor 50 via the relevant input terminal so that a predetermined motor current (e.g. 25 amperes) can flow. At that time point, the current flowing through the steering effort assisting motor 40 is actually measured by means of the measuring unit.
Further, the motor current adjusting unit is so designed as to perform the adjustment of the motor current by selecting sequentially the values of the adjusting resistor RA incorporated in the motor current detecting means 48 so that the actually measured motor current value as measured by the measuring unit lies within a predetermined range (e.g. .+-.1 ampere) relative to the predetermined motor current (25 amperes).
FIG. 9 is a structural diagram showing a conventional control system for a motor vehicle which includes an internal adjusting mechanism, and more specifically shows an engine control system designed for protecting an exhaust gas system in response to abnormality of the exhaust gas temperature.
Referring to FIG. 9, an exhaust gas temperature sensor 100 designed for detecting the temperature of the exhaust gas of the engine functions as an input unit for a control unit 101. To this end, the exhaust gas temperature sensor 100 may be constituted by a thermocouple such as e.g. chromel-alumel thermocouple (hereinafter referred to also as the CA in abbreviation).
The output signal of the exhaust gas temperature sensor 100 is inputted to a control unit 101 which incorporates a microcomputer 300 as a control means.
The control unit 101 is comprised of an amplifier 200 which is designed for amplifying the output signal of the exhaust gas temperature sensor 100 before inputting it to the microcomputer 300 and which per se is known in the art, a resistor R21 inserted at the input side of the amplifier 200, an adjusting resistor RA1 connected in parallel with the resistor R21, resistors R11 and R12 for determining the gain G of the amplifier 200, and an alarm driving circuit 400 inserted at the output side of the microcomputer 300.
An offset voltage Ve of the amplifier 200 can be adjusted with the aid of the resistor R21 and the adjusting resistor RA1.
The microcomputer 300 includes an A/D (analogue-to-digital) converter 310 for converting the output signal of the amplifier 200 into a corresponding digital signal and a CPU (central processing unit) 320 to which the output signal of the A/D converter 310 is inputted.
The alarm driving circuit 400 can be implemented, for example, by a power transistor and serves as an output control unit (output interface) for the microcomputer 300. The alarm driving circuit 400 responds to the output signal of the microcomputer 300 to drive an alarm lamp 500 connected to the control unit 101. The alarm lamp 500 serves as an output unit for the control unit 101.
Next, description will be directed to operation of the conventional control system for the motor vehicle shown in FIG. 9.
In general, a voltage level of the output signal of the exhaust gas temperature sensor 100 which is constituted by the CA is only on the order of ca. 45 mV for the temperature difference of 1200.degree. C. from a reference point.
On the other hand, the LSB (least significant bit) of the A/D converter 310 incorporated in the microcomputer 300 is about 19.5 mV with resolution of 8 bits and about 4.9 mV with resolution of 10 bits when operated at an ordinary voltage level of 5 volts.
Accordingly, unless the detection value of the exhaust gas temperature is amplified, the microcomputer 300 is only capable of detecting the temperature by a scale unit of 130.degree. C. even with the resolution of 10 bits. As a result of this, even when the alarm lamp 500 is so set that it can not be lit under proper conditions, abnormality of the exhaust gas temperature will be detected, making it impossible to realize the protection of the exhaust gas system.
Under the circumstances, the amplifier 200 is provided for ensuring a sufficiently high detection resolution, as shown in FIG. 9. In this conjunction, when a commercially available operational amplifier, for example, is used as the amplifier 200, an input offset voltage Ve of ca. 7 mV at maximum makes appearance, involving a detection error of 187.degree. C.
Thus, with a view to compensating for the offset error of the operational amplifier (amplifier 200), such an arrangement is heretofore adopted that the adjusting resistor RA1 can be inserted in each control unit 101 (refer to FIG. 9).
When adjustment is performed in the arrangement shown in FIG. 9, the offset error is first measured in the state in which the resistor R21 is connected actually, whereon the appropriate resistance value of the adjusting resistor RA1 is arithmetically determined and then the adjusting resistor RA1 set to the resistance value as determined is connected in parallel to the resistor R21.
However, because the adjusting resistor RA1 generally exhibits discrete resistance values, it is practically impossible to realize the adjustment with high accuracy.
Furthermore, a space for accommodating the adjusting resistor RA1 is required to be reserved in advance. Besides, a step for connecting the adjusting resistor RA1 is additionally involved. These factors thus provide causes for increasing the manufacturing cost of the control unit 101.
Of course, the adjusting resistor RA1 may be constituted by a semi-fixed type variable resistor or alternatively by a resistor film deposited on a ceramic chip so that it can be trimmed by using a laser beam. In that case, however, not only the resistor itself is of high price but also expensive adjusting device is required, not to say of a lot of time taken for the adjustment. Consequently, the manufacturing cost of the control unit 101 will further be increased.
Additionally, it is noted that with the conventional arrangement, the number of adjusting parts increases substantially in proportion to the number of adjustments, which is of course accompanied with increase in the time taken for the adjustment. Thus, the manufacturing cost increases remarkably, to a great disadvantage.
As is apparent from the foregoing, in the conventional control systems for the motor vehicle known heretofore, adjustment is carried out by measuring the motor current IM flowing through the steering effort assisting motor 40 in the state in which only the resistor R1 constituting a part of the motor current detecting means 48 is mounted, whereon the adjusting resistor RA having been set to the proper resistance value as selected is connected in parallel with the resistor R1 in the system shown in FIG. 7 and FIG. 8.
Similarly, in the case of the system shown in FIG. 9, the offset error is measured in the state in which only the resistor R21 constituting a part of the control unit 101 has been mounted, whereon the adjusting resistor RA1 having an appropriate value determined arithmetically is connected in parallel with the resistor R21.
However, in any one of the cases mentioned above, the adjusting resistors RA and RA1 exhibit discrete resistance values. Consequently, the adjustment can not be realized with desired accuracy, giving rise to a problem.
Besides, because a space for mounting the adjusting resistor RA or RA1 has to be secured in advance, the apparatus will have to be implemented in a large size. Further, an additional step is required for the connection of the adjusting resistor RA or RA1. Thus, the manufacturing cost is increased, giving rise to another problem.
Moreover, the device for the adjustment is intrinsically very expensive and requires a lot of time for the adjustment, which ultimately results in increase of the manufacturing cost of the control system as a whole, giving rise to yet another problem.
The present invention has been made in an effort to solve the problems such as mentioned above and contemplates as an object to provide a control system for the motor vehicle which is equipped with an inexpensive and precise adjusting means.